Abstract
The relative distribution of chlorophyll (Chl) a and Chl b, of Photosystem I (PS I), and of the cytochrome b 6- f (Cyt b 6- f) complex in different subchloroplast membrane vesicles is presented. Spinach chloroplasts ( Chl a Chl b = 2.7 , Chl PS I = 435:1 , Chl Cyt f = 970:1 ) were fractionated by Yeda press treatment and by sonication. Counter-current distribution of the resulting thylakoid vesicles in dextran-polyethylene glycol aqueous two-phase systems yielded a laterally extended range of chloroplast thylakoid membranes without loss of chloroplast material from the initial sample. In this analysis, PS I-enriched membrane vesicles moved in the leading edge of the counter-current distribution diagram, whereas PS II-enriched vesicles remained close to the trailing edge of the counter-current distribution diagram. Vesicles of mixed composition and vesicles depleted in both PS I and PS II were isolated from intermediate positions. The Chl PS I ratio in the various fractions approximated a hyperbolic function of the Chl a Chl b ratio with limiting values of Chl PS I = 190:1 attained at a Chl a Chl b = 6 and a Chl a Chl b = 1.67 attained when the Chl PS I ratio approaches infinity ([PS I] = 0). Such hyperbolic distribution is consistent with the exclusive localization of PS I in stroma-exposed thylakoids and of PS II in the membrane of the grana partition region. A depletion of the Cyt b 6- f complex from the membrane domain of PS I was detected ( Chl a Chl b = 5.1 , Chl PS I = 210:1 , sol Chl Cyt f = 1510:1 ). Moreover, a chloroplast thylakoid membrane was identified, distinct both from that of PS II and from that of PS I which was enriched in the Cyt b 6- f complex ( Chl a Chl b = 2.67 , Chl PS I = 460:1 , Chl Cyt f = 590:1 ). Our results suggested that such a Cyt b 6- f domain is localized in the region between the appressed and the non-appressed membranes, possibly in the membrane of the fret region.
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